Geostationary Communication Satellites
Satellite communications utilizes electromagnetic waves to carry information from the ground to space and back. An electromagnetic wave consists of an electric field and a magnetic field that are perpendicular to each other and to the direction of propagation. Polarization is a term that defines the orientation of the electric field as the wave propagates through space. It can be manipulated into two commonly employed types of polarization: Linear (e.g. vertical, horizontal and slanted) and Circular (Right-Hand and Left-Hand) polarizations.
An important application of polarization of the signals broadcast is in frequency reuse. Polarization of the broadcasts of two electromagnetic waves, one traveling in the vertical plane and the other in the horizontal plane, allows both broadcasts to use the same frequency without unduly impacting one another. This provides the ability to essentially double capacity of frequencies available for use.
The term earth station is the internationally accepted term that includes satellite communications stations located on the ground. They can be configured and utilized in a number of ways, but in order for an earth station to transmit or receive a signal, it will require uplink and/or downlink equipment. At both ends of the communication link between the earth station and the satellite, an antenna linked to a transponder provides both the means to transmit the radio frequency (RF) signal to the satellite and to receive a signal from the satellite. Ideally, antennas for this purpose help to minimize Radio Frequency interference (RFI) by using reflectors to focus the RF signal onto a single satellite.
Commercial geostationary communication satellites typically employ linearly polarized signals; however, some also employ circular polarization. The transponder polarization is defined at the satellite with a “horizontally” polarized signal having its E-field oriented parallel with the equatorial plane and a “vertically” polarized signal having its E-field oriented perpendicular to the equatorial plane (or parallel to the Earth's rotational axis).
Since a geostationary satellite in general may not be at the same longitude as an Earth station, the polarization of the satellite signals as viewed from the Earth station will usually not correspond to horizontal and vertical in local Earth station coordinates. If the satellite longitude is far to the east or west of the Earth station, the signal polarization as viewed at the Earth station may differ substantially from the nominal polarization defined at the satellite. This difference may approach 90 degrees when the satellite is near the horizon and the Earth station is at a low latitude. Since the available satellites are stationed at different longitudes, the apparent polarization slant angle will vary from satellite to satellite.
As noted, to achieve maximum spectral usage of the limited spectrum available, completely independent signals are transmitted and received by the satellites on orthogonal polarizations, typically designated “horizontal” (H) and “vertical” (V), on the same frequency. This practice of transmission or reception of independent signals on the two polarizations is called “frequency reuse.” Since frequency reuse provides a substantial economic benefit, it has become the standard for nearly all geostationary commercial communication satellites. However, frequency reuse requires that the earth station polarization be accurately aligned with the satellite polarization. More importantly, it also requires the earth station to have excellent rejection of the undesired polarization on both the uplink (transmit) and downlink (receive) sides of the communication link to prevent interference to or from other users of the same satellite. For this reason, Earth stations must provide a capability for adjusting their transmit and receive polarizations to closely match those of the satellites with which they communicate.
Conventional Earth stations employ reflector (“dish”) antennas which typically use a circular feed horn with an orthomode coupler or “transducer” (OMT) to implement the two orthogonal linear polarizations (for transmit and receive). The feed horn is mechanically rotated to precisely match its polarizations with those of the satellite signals. The circular feed horn/OMT is a relatively simple device that has little impact on the overall design of the reflector antenna.
In future satellite communication applications it may be desirable to replace the reflector antenna with a phased array. Phased array antennas employ a plurality of “radiator” elements and their associated active electronics to form a beam for transmission or reception. The beam is pointed or scanned electronically by means of phase control devices associated with each radiator element. Thus, a phased array can provide beam pointing and/or scanning without the use of moving parts. Sidelobes are typically controlled by means of amplitude weighting applied through amplitude control devices associated with each radiator element. A phased array can therefore provide more flexibility and capability in controlling sidelobes than a reflector antenna. These principles are well-known and well-documented in the literature, e.g., Mailloux (1994) and Hansen (1998).
The beam pointing and sidelobe control functions require control of the phase and amplitude of the RF signals passing through each radiator element (radiator) in the phased array. (The term “radiator” as employed herein is used for both receive elements and transmit elements). The active electronic circuits associated with each radiator element are often collectively referred to as a “channel” or a “module” (e.g., transmit module, receive module, T-R module) although these electronic circuits may physically be grouped together into larger assemblies. FIGS. 1-1a through 1-1c show typical overall array architectures for transmit (TX), receive (RX) and transmit-receive (T-R) phased arrays, respectively.
Polarization in phased array antennas must be controlled at the element level and ideally should be fully electronic. This makes the problem of polarization control in phased arrays more complex than the above noted case of polarization control in reflector antennas. One approach used in the prior art involves a dual-polarized radiating element driven by separately-controlled excitation signals for the two orthogonal polarizations. By adjusting the amplitude and phase differences between the two excitations any polarization state may be achieved. Completely independent amplitude and phase control for the two polarizations also facilitates measurement and correction of errors, a process known as calibration.
Limitations of the Prior Art
As FIGS. 1-2a and 1-2b indicate, implementing full polarization agility in a transmit or receive module essentially doubles the number of active components required with respect to the number in a single-polarization module. This has a very significant impact on the array cost and power consumption/dissipation per element. It may also increase the difficulty of implementation at high microwave frequencies where the space for components behind each radiator element is limited.
It would be desirable to implement a polarization control scheme which introduces a minimal amount of additional complexity above that required for a single-polarization phased array. Such an approach would not only reduce the RF parts count per element but would also simplify the digital control system since approximately half the number of command bits per element would be required. The disclosed approach to polarization control enables the element-level electronics to be simplified from two signal paths or “channels” in the prior art (as in FIGS. 1-2) to a single channel as shown in FIGS. 2-1 through 2-3. This has a number of benefits, which are objects of this invention including:    1. Significantly reduced cost of the element-level electronics due to the reduced parts count while retaining full polarization control in the main beam.    2. Reduced space/volume required by the element-level electronics, permitting polarization control in arrays at high microwave frequencies where close element spacing may not permit the use of two channels per element.    3. Reduced power consumption.    4. Reduced thermal dissipation.    5. Simpler control interface circuit configuration.    6. Reduced throughput requirements in the control interface.    7. Reduced throughput requirements in the beam steering controller.    8. Smaller calibration tables.
Another object of this invention is to provide an improved method for control of polarization of a phased array antenna.
An additional object of this invention is the provision of a method for configuring a phased array antenna for angle and polarization which employs a novel polarization assignment algorithm.
Another object of this invention is the provision of such a control scheme for polarization of a phased array antenna which introduces a minimal amount of additional complexity above that required for a single-polarization phased array.
An additional object of this invention is the provision of such a control scheme for polarization of a phased array antenna which minimizes the cost and complexity of implementation.
Yet another object of this invention is to provide a method of dynamically allocating the individual polarization of radiator elements between their individual horizontal and vertical polarization modes, to yield the desired slant angle for a phased array antenna.
With respect to the above description, before explaining at least one preferred embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangement of the components or steps set forth in the following description or illustrated in the drawings. The various apparatus and methods of the invention are capable of other embodiments and of being practiced and carried out in various ways which will be obvious to those skilled in the art once they review this disclosure. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
Therefore, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing of other devices, methods, steps, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the objects and claims be regarded as including such equivalent construction and methodology insofar as they do not depart from the spirit and scope of the present invention.
These together with other objects and advantages which become subsequently apparent reside in the details of the construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part thereof, wherein like numerals refer to like parts throughout.